A conductive-bridging resistance-change memory element, which will be referred to herein as a resistance memory element, has at least two distinct stable resistance states: a low-resistance state and a high-resistance state. A resistance memory element can be switched from a high-resistance state to a low-resistance state by the application of a voltage pulse of one polarity and can be switched from a low-resistance state to a high-resistance state by the application of a voltage pulse of the opposite polarity. Each resistance state is used to represent a respective data value, e.g., 1 or 0. Since the resistance changes can be accomplished by applying low-voltage pulses to the resistance memory element, and the resistance state of the resistance memory element can be determined by applying a voltage or a current to the resistance memory element, resistance memory elements are attractive for use in low-cost, high-density memory arrays.
FIG. 1 depicts the basic current-voltage characteristics of an exemplary resistance memory element. Initially, the resistance memory element is in a high-resistance state. Applying a positive voltage pulse having a magnitude greater a first threshold voltage that will be referred to herein as a SET voltage to the resistance memory element causes the resistance of the resistance memory element to drop by several orders of magnitude and a corresponding increase in the current through the resistance memory element. In an example, the SET voltage is about 250 millivolts (mV). Applying a positive voltage pulse having a magnitude greater than the SET voltage changes the resistance memory element from a high-resistance state that will be referred to as a reset state to a low-resistance state that will be referred to herein as a set state. The high-resistance state of a resistance memory element will be referred to herein as a reset state and the low-resistance state will be referred to as a set state. The names of the states can be interchanged.
Applying a negative voltage pulse having a magnitude greater than a second threshold voltage that will be referred to herein as a RESET voltage causes the resistance of the resistance memory element to increase by several orders of magnitude and a corresponding decrease in the current through the resistance memory element. In an example, the RESET voltage is about −80 mV. Applying a negative voltage pulse having a magnitude greater than the RESET voltage changes the resistance memory element from the low-resistance set state back to the high-resistance reset state.
Application of a read pulse having a voltage less than the SET voltage to the resistance memory element causes a read current to flow through the resistance memory element. As shown in FIG. 1, the read current differs significantly depending on whether the resistance memory element is in the low-resistance set state or in the high-resistance reset state. By measuring the read current (or comparing the read current with a threshold), the resistance state of the resistance memory element can be detected and, hence, the data represented by the resistance state of the resistance memory element can be read.
In a memory array composed of resistance memory elements, parasitic currents can pass through unselected resistance memory elements. Such parasitic currents can make it difficult or impossible to determine whether the measured read current represents the high-resistance state or the low-resistance state of the resistance memory element that was selected to be read. Since the read current is used to determine the data value stored in the resistance memory element, it is consequently difficult to determine the value of the stored data. To alleviate this problem, access devices are used to select the resistance memory element to be read. The access devices are used to suppress or minimize the parasitic currents that flow through unselected resistance memory elements. However, the use of an access device alters the current-voltage behavior of a resistance memory cell that incorporates a resistance memory element and an access device.